1. Field of the Invention
The present invention relates to an illuminance calibrating method of an illuminator, illuminance calibration controller of an illuminator, illuminance calibrating program of an illuminator, recording medium storing the program and a measuring tool. For instance, it relates to an illuminance calibrating method of an illuminator that calibrates an illuminance of an illumination light irradiated by an illuminator having a light source capable of changing the illuminance thereof in accordance with applied current value.
2. Description of Related Art
Image-processing measuring tools having an illuminator that irradiates a light beam toward a workpiece, an image pickup device that receives the light reflected by the workpiece, and an image processor that obtains the profile of the workpiece by processing the image received by the image pickup device have been known.
The illuminator has a plurality of light sources, an current controller that controls the electric current applied to the respective light sources, and an input device capable of being operated from the outside to input the illuminance of the illumination light as a command value.
In the above device, a user commands the illuminance of the illumination light by an input device. Then, the current controller applies a predetermined electric current to the respective light sources so that the illumination light of a commanded illuminance is irradiated therefrom. By applying the electric current, the illumination light of the commanded illuminance is irradiated on the workpiece. The light reflected by the workpiece is received by the image pickup device and the profile and/or dimension of the workpiece is measured by detecting the edges of the imported image.
In the image measurement, the illuminance of the illumination light irradiated on the workpiece is of extreme importance. For instance, when the actual illuminance of the illumination light is low relative to the commanded illuminance, the image cannot be taken or the edge of the image cannot be detected on account of insufficient luminous energy. On the other hand, when the actual illuminance is high relative to the commanded illuminance, the light is saturated and the image is blurred (i.e. over-illuminated) so that the edge cannot be detected.
As described above, accurate control of the illuminance of the illumination light by controlling the electric current applied on the respective light sources is extremely important for precise measurement.
However, since the light sources have individual differences for each product, the illuminance differs for each individual product even when the same electric current is applied as specified therefor. Then, the illuminance of the illumination light becomes different for each illuminators even when the same electric current is applied.
In view of the above, an illuminance meter for measuring the illuminance of the illumination light is used to measure the relationship between the applied electric current and the illuminance of the illumination light. By calculating the applied current value required for obtaining commanded illumination light based on the data, the relationship between the applied current value and the illuminance is calibrated for each illuminator.
However, an illuminance meter is very expensive, which amounts to a couple of hundred thousand yen (three hundred thousand yen, for instance). A lot of cost is requested for a user if he has to purchase such an illuminance meter in order to calibrate the illuminance of the illuminator.
Further, the illuminance meter requires a couple of seconds of time for one measurement. A huge amount of time is required for measuring the relationship between the applied current value and the illuminance by a couple of hundreds or a thousand steps.